In a nuclear power plant, when radioactive substances are chemically removed from apparatuses and pipes of a primary cooling system contaminated by radioactive substances and from surfaces of metal members of the system including those mentioned above, a large amount of decontamination waste liquids is generated. Those decontamination waste liquids contain iron-group metal ions of Fe, Co, or Ni and also contain a large amount of radioactive substances, such as Co-60 (cobalt 60) and Ni-63 (nickel 63). In general, a decontamination waste liquid is reused after ion components dissolved therein are removed by an ion exchange resin as a decontaminated liquid. Hence, there has been a problem in that a waste ion exchange resin containing a large amount of radioactive substances is generated.
In a nuclear power plant and the like, since an ion exchange resin used for cleanup of a cooling water system, such as a reactor water cleanup system (CUW) or a fuel pool cooling cleanup system (FPC), which is directly brought into contact with a fuel rod and contains radioactive substances adsorbs a large amount of radioactive substances, as a high-dose rate waste, the above ion exchange resin is stored in a resin tank provide in the power plant.
Those wastes containing radioactive substances are stabilized by kneading with a solid-forming auxiliary agent, such as cement, and finally, burial disposal thereof is performed. The cost for the burial disposal is changed depending on the amount of contained radioactive substances and is increased as the concentration thereof is increased. Hence, it is economical that after the volume of a high-dose rate waste is reduced as much as possible, a solid waste for burial disposal is formed. In particular, if the radioactive substances can be isolated in a solid form from the ion exchange resin and can be sealed in a shielding container, it is preferable in terms of the reduction in volume. Since a waste ion exchange resin from which the radioactive substances are removed is a low-dose rate waste which can be disposed at a low cost, if the radioactive substances can be removed therefrom to a level at which the waste ion exchange resin can be incinerated, a significant reduction in volume can be achieved by an incineration treatment.
As a treatment method of a high-dose rate waste resin as described above, as proposed in Patent Literature 1 and Patent Literature 2, a Fenton method and a method for decomposing a waste resin by wet oxidation, such as supercritical water oxidation, have been known. When the methods as described above are used, in both the cases, a large amount of a high-dose rate waste liquid is generated. When this high-dose rate waste liquid is finally disposed, after evaporative concentration thereof is further performed, the concentrated liquid thus obtained is required to be stabilized in a solid form, for example, by a method for kneading the liquid with cement. In this case, since a solid-forming auxiliary agent, such as cement, is newly added, the volume of a high-dose rate waste to be finally disposed is increased by an amount corresponding to that of the agent, and as a result, a problem in that the reduction in volume of the waste cannot be achieved may arise.
Patent Literature 3 has disclosed a technique in which after sulfuric acid is allowed to pass through a waste resin to elute ionic radioactive substances therefrom, the radioactive substances are isolated from the eluent by diffusion dialysis, and the sulfuric acid is recycled. Patent Literature 4 has disclosed a waste resin treatment method in which a waste resin is immersed in an oxalic acid aqueous solution to dissolve a metal clad on the surface of the resin, and in addition, metal ions adsorbed to the resin are also eluted into the oxalic acid aqueous solution. In the cases described above, although a waste liquid containing radioactive substances is produced, the solidification treatment thereof has not been sufficiently described.
As a method for removing radioactive substances from a waste liquid containing ionic radioactive substances, Patent Literature 5 has disclosed a technique for regenerating and reusing a decontamination solution in which while a decontamination solution dissolving radioactive cations is allowed to pass through an electrodeposition cell, voltage application is performed thereon to deposit the radioactive cations on a cathode as radioactive metal grains. In this case, it has been described that a cathode liquid is pored over the entire cathode so that the radioactive metal grains are removed from the cathode on which the radioactive metal grains are deposited.
In Patent Literature 5, while the decontamination solution dissolving radioactive cations is directly charged to a cathode side of the electrodeposition cell, by applying the voltage thereon, the radioactive cations are deposited on the cathode as the radioactive metal grains. In this method, since the cathode liquid properties are changed depending on the decontamination solution, the cathode liquid cannot be adjusted to have liquid properties suitable for electrodeposition. When the decontamination solution is an acidic waste liquid, since a radioactive metal component precipitated on the cathode surface is again dissolved in the acidic waste liquid, precipitation may not occur, or the precipitation rate may be seriously decreased. When the waste liquid is neutral or alkaline, a hydroxide deposit is formed in the vicinity of the cathode surface, and the recovery of the radioactive metal by electrodeposition thereof on the cathode surface becomes difficult. Hence, in order to efficiently recover radioactive substances from a waste liquid by an electrodeposition method, direct charge of a waste liquid into a cathode chamber is not preferable, and it is important to adjust the cathode liquid to have liquid properties suitable for electrodeposition.
In addition, in order to efficiently recover radioactive substances from a waste liquid by an electrodeposition method, it is significantly important to appropriately select the liquid properties of a liquid into which the cathode is immersed.
In a nuclear power plant, since an ion exchange resin used for cleanup of a cooling water system, such as a reactor water cleanup system (CUW) or a fuel pool cooling cleanup system (FPC), which is directly brought into contact with a fuel rod and contains radioactive substances adsorbs a large amount of radioactive substances, as a high-dose rate radioactive waste, the above ion exchange resin is stored in a resin tank provide in the power plant. In a nuclear power plant, when radioactive substances are removed by chemical cleaning from apparatuses and pipes of a primary cooling system contaminated by radioactive substances and from surfaces of metal members of the system including those mentioned above, an ion exchange resin is also used, and the ion exchange resin thus used is also stored in a resin tank as a high-dose rate radioactive waste. Those wastes containing radioactive substances are stabilized by kneading with a solid-forming auxiliary agent, such as cement, and finally, burial disposal thereof is performed. The cost for the burial disposal is changed depending on the amount of contained radioactive substances and is increased as the concentration thereof is increased. Hence, it is economical that after the volume of a high-dose rate waste is reduced as much as possible, a solid waste for burial disposal is formed. In particular, if the radioactive substances can be isolated in a solid form from the ion exchange resin and can be sealed in a shielding container, it is preferable in terms of the reduction in volume. Since a waste ion exchange resin from which the radioactive substances are removed is a low-dose rate waste which can be disposed at a low cost, if the radioactive substances can be removed therefrom to a level at which the waste ion exchange resin can be incinerated, a significant reduction in volume can be achieved by an incineration treatment.
When a waste resin can be treated by incineration disposal, although a significant reduction in volume of radioactive wastes can be achieved, in this case, the radioactive substances are concentrated in incinerated ash, and hence, the incinerated ash becomes a high-dose rate material. If the radioactive substances can be completely removed from the waste resin, the incinerated ash can be prevented from becoming a high-dose rate material, and the reduction in volume can be performed by incineration; hence, various techniques for removing radioactive substances from a waste resin have been investigated.
A high-dose rate waste resin used in a reactor water cleanup system or a fuel pool cooling cleanup system adsorbs ions of radioactive substances and also contains a clad primarily formed of iron oxide. Since the clad also contains radioactive substances, in order to completely remove radioactive substances from the waste resin, the clad is also required to be simultaneously removed from the waste resin.
As the chemical form of the clad contained in the waste resin, magnetite (Fe3O4) and hematite (α-Fe2O3) are primarily present. As a technique for removing radioactive substances from a waste resin, in Patent Literature 6, a technique has been disclosed in which after sulfuric acid is allowed to pass through an eluting device in which a waste resin is packed to elute ionic radioactive substances therefrom, from the eluent, the radioactive substances are isolated by diffusion dialysis, and the sulfuric acid is recycled. As described above, in the method in which a room-temperature sulfuric acid which is not heated is allowed to pass through a waste resin, since poor soluble hematite (α-Fe2O3) is difficult to be dissolved, and the clad cannot be remove from the waste resin, a problem in that radioactive substances remain may arise in some cases.